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SETI question

  1. Feb 19, 2006 #1
    I have a question about radio waves.

    If there was an ET civilization out there broadcasting in the EM spectrum exactly like we do, how far away could it be where we would still be able to detect it? (naturally this assumes it transmitted at the proper time for us to receive the signal, ie 40 years ago if it is 40 light years away).

    I am just curious as to how far away a civilization like us could be and still be able to detect OUR signals given that they have similar capabilities. I realize that an advanced civilization may be much better at it than we are.
  2. jcsd
  3. Feb 19, 2006 #2


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    It depends on how strongly they send the signal, and how sensitive our instruments are.
  4. Feb 19, 2006 #3
    good answer

    Ya think? Did you come up with that off the top of your head or did you have to research it?
  5. Feb 19, 2006 #4


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    He gave a perfectly good answer to your question. If that was not what you intended, rephrase your question.
  6. Feb 19, 2006 #5


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    it lacks a little. assuming the questioner does not already have all of the answers to rephrase it, there are other issues. there is extracting the signal out of noise, i.e. it might have been transmitted with enough power to be detectable with receivers as sensitive as we can make, but if it's smothered in space noise, even the most sensitive receiver will not be able to extract that signal out of noise that is so much louder.

    now there are technological solutions (spread spectrum and/or error correcting coding), but it requires that ET would not only be thinking of them (for our benefit) but that ET and us could sorta agree on the format.

    the Voyager deep space probes had a 20 watt transmitter and we still got these beautiful photos from them from distance of billions of km. if you work out what the inverse-square law does to that (i dunno the directivity of the antenna, but i don't think it's as much as 40 dB, but what do i know?), what makes it to the Earth is not very powerful. but we had multiple antennas all over the side of the planet facing Voyager and they were all pointing in the right direction and they were carefully synchronized. the component of the received signal that is noise will sorta tend to cancel (actually the noise powers will add) because much of be out of phase but, if they get the sychronization down tight, the compoent of the received signal that is the transmitted signal will team up (the signal voltage will add which is better than if the signal power adds).

    now that is "spread spatialization" (my made up term) with a synchronized signal received at multiple places but the noise is dumb and will not synchronize itself to all of the receivers. now imagine doing that with added spectrum (transmitting a redundant signal over many different frequency bands) and with added time (transmitting a redundant signal over many different slices of time). the transmitter in Voyager and receiver on Earth agreed to a format to send and collect this data in a coded and redundant way and, as a result, we got some pretty nifty pictures including one of the little blue dot that we live on.

    so what we gotta do is think about how ET is gonna send that data to us without ever meeting with him in a "convention" to nail down the specs and how it's gonna be coded and modulated and at what frequencies it will be transmitted. dunno how they're gonna do it but people write papers about it. (like, if we wanted to send data to ET, how are we gonna format it in such a way that ET could be expected to recognize it over the noise and would logically be able to figure out how to decode it?)

    all this is doable, but figuring out the most logical way to do it so that ET might be expected to do it the same way requires a lot of thought. (we presume that ET can figure out Planck Units, understands the fundamental nature of base-2 numbering, and knows about Hydrogen and other elements and the frequency of radiation one can get out of them so we have some good guesses about where in the spectrum to listen.)

    i just noticed this form of the question:
    if ET is broadcasting music or whatever in AM or wideband FM with 50 kW or whatever transmitters, there is no chance at all that we will be hearing that unless we construct a huge array of antennas in space with an aperature a helluva lot bigger than the Earth. and then we would need to know what direction to point it (that is also an issue of picking up deliberate transmissions of ET to us).
    Last edited: Feb 19, 2006
  7. Feb 19, 2006 #6
    You are right, it was a perfectly good answer. If I were a 5 year old. Since I'm not, pointing out the obvious lacks quite a bit in even coming up to the barely adequate level.

    My question, after rereading it several times, is fine as is. But I will try to clarify it a little.

    I am looking for an answer in light years as to how far away OUR present detection methods (I am assuming radio telescopes) could detect OUR signals.

    I am asking this because every year our earliest signals go out another light year and I have no idea whether they can be detected no farther than 50 or 50,000 light years.
  8. Feb 19, 2006 #7


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  9. Feb 20, 2006 #8


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    it's a cute and somewhat informative article. but i would like to see them quantitatively justify the conclusion:

    my dubiousness comes from the fact that there is rough isotropy within 100 lightyears so the astronomical background noise does not suffer inverse-square loss but the signals transmitted from earth do suffer inverse-square attenuation.

    now when i look it up, it looks like for just the Big Bang background radiation the intensity is about 1.2 x 10-4 (erg/s)/(cm2 sr c/cm) or about 4 x 10-18 (W/m2)/(sr Hz) and that does not include the crap from stars and local celestial objects, does it?

    now let's see how much a 100 kW radiating omnidirectionally will get at,say 50 lightyears: (105 W)/(4 pi (50 1016m)2) or about 10-31 W/m2. if the signal bandwidth is 10 kHz, the signal-to-noise ratio (S/N) is 10-31/(104 10-18) or 10-17 or -170 dB. yanking a signal out of that is a big deal.

    looks like i dropped the steradians of your aperature. how small can that be with a radio telescope? i can't imagine much less than say 10-6 steradian, but i don't know. that would help reduce noise by 60 dB, but then you would have to know right where to point the thing.

    at some other time, i read that receiving regular broadcast from interstellar distances would require an array of planetary size and seeing the numbers to justify that. that's not the same as receiving a narrow beam (using a radio telescope dish to transmit), narrow bandwidth (so much for spread spectrum), very high wattage transmission. i think the issue is focusing the receiving aperature to be very small, which like optics, requires a big lens.

    someone else can jump in with numbers to solve this.
  10. Feb 20, 2006 #9


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    i guess my other dubiousness is that the alien transmitter will be on or in orbit of an alien planet which itself is orbiting a star and, from our POV, will be inseperable from the star. no matter how tightly we can focus our radio telescope to leave out the cosmic radiation from all directions other than where the aliens (and their star) are at, we still have to soak in the radiation of the star. how can any mortals expect to make a transmitter than can compete with the output of a star?
  11. Feb 20, 2006 #10


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    We do it all the time. The Sun's radio frequency emissions deviate quite significantly from blackbody (related to particle energies and magnetic fields). The blackbody portion of the emission is generally VERY weak (plug values into the Planck radiation formula). In either case, you're looking for a coherent signal above background levels. We (or aliens) can certainly broadcast coherent signals at intensities above thermal levels and distinguish them from cyclotron, synchrotron, upper or lower hybrid and other kinds of stellar emissions related to magnetic fields.
  12. Feb 20, 2006 #11


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    You asked a simple question and you got a simple answer. Now for your new question. We can't really detect our own signals.. it's like asking if a pitcher could catch his own fastball.

    As for your new question: Our earliest signals can only be detected up to say, 80 light yeras away because probably the earliest signals sent were in the 20's or 30's.
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